Electrical compressor method and system



ELECTRICAL COMPRESSOR METHOD AND SYSTEM Filed June 29. 1942 Maxim/m Oes/'rab/e eve/ ATTQRNEY.

Patented Feb. 19, 1946 UNITED 'sTATEsi PATENT OFFICE l.

ELECTRICAL COMPRESSOR METHOD AND SYSTEM Frederick G. Albin,- Los Angeles, cam., assigner tc- Badio Corporation of of Delaware Amex-ica, a corporation Application June as, 1942, serial No. 44.8.9116v s claims. (ctms-1004) This invention relates to electrical current transmission systems and particularly to an amplifier or attenuator arrangement for` varying the ratio of the amplitudes reproduced with respect to those' of the original signal.

Although. the invention is useful in broadcast and public address systems where compression is desired for various reasons. it is particularly useful in sound recording, wherein the' sound track area of a film has denite physical limitations and can, therefore, accommodate only a certain amplitude range which isvgenerallyless than the amplitude range of the sound source. Thus, for this reason and others compression is introduced in systems for recording sound on film, a standard type of sound recording compressor being shown in Singer Patent No. 2,255,683 of September 9, 1941. In this patented system a portion of the output of a variable gain amplier is rectied and impressed upon the gain control elecwhich compression begins being controlled as well level. Thus, compression as used in the present manner is acceptable. y On the other hand, a musical recording such D as a solo with accompaniment,`cons ists of widely separated frequency bands. An example is aA soprano voice, or iiute or other high register in- V strument with accompaniment by a low register trodes of the amplifier, the` amp1itude level 'at I instrument such as a guitar, double bass viol, etc. In this case, the ear of the auditor may easily recognize the individual power levels of the solo and the accompanying instrument.

When a single compressor is used in the recording system, the level of the solo frequency is compressed at a rate determined by its level. but at the introduction of the added power from the accompanying instrument, the solo frequency suffers added compression in accordance with the power level of the solo frequency` plus the power level of the accompanying instrument. Frequently the level from the accompanying instrument may be the larger and produce a serious fluctuation of the solo frequency level, which flucas the rate or amount of compression obtained for any particular input level.

The present invention is directed to a compressor system of this general type, but one wherein the signal is dividedinto mutually exelusive frequency bands to which are applied difa ferent rates of compression. VThewbreak-away amplitude levelat which compression begins is also individually controlled to provide compres` sion in accordance with frequency. In this manpermits controlling this ratio to the optimum ex ent.

The present use of`compressors in the recording of dialogue or music is such that the-compression of the composite sound level is the/result of the level of the composite level. In any composlte sound level, the power levels of the individual frequencies add. together arithmetically toproduce the composite power level. yIn the case of speech, the frequency components for the larger part lie in the same band, and the ear does not differentiate between frequencies whenV Judging level, but recognizes only thecomposite tuation is clearly distinguishable by the auditor.

It is the avoidance of this interacting eect which is an important feature of this 4invention, wherein the widely separated frequencies are passed through separate compressor branches where each is compressed in accordance with itsown level, and independently of the level in the other branch or branches. l

'Ihe principal object of the invention, therefore, is to provide improved means for facilitating 'the compression of signals being transmitted. Another object of the invention is to provide an improved compression system which not only produces a predetermined amount of compression of a given signal, but varies the compressed amplitude ratio in accordance with frequency.

A furtherpobject of the invention is to provide improved means for facilitating the compression of signals being recorded on a sound track.

A further object of the invention is to provide an' improved compressor system for sound recording wherein the signal range is compressed within the nlm range and the ratio of high to low frequencies is varied with signal amplitude.

Although the novel features which are believed to be characteristic ofthisinvention are pointed out with particularity in the claims appended herewith, the manner of its organization and the mode of its operation will bebetter understood` by referring to the following description;` read in conjunction with the accompanying drawing, in which:

55 Fig. 1 is a diagrammatic arrangement of a y sound recording system embodying the invention;

Fig. 2 is a modification of the system of Fig. 1;

and

Fig. 3 is a graph showing the variation of high to low frequencies with signal level.

Referring now to Fig. 1 in which a sound rey cording system is shown diagrammatically, a e microphone 5 feeds an amplifier 5, the output of which is impressed upon a low-pass filter 1 and high-pass lter 8. The output ofthe low-pass filter is fed to a variable gain amplifier I with its associate control unit II and then to a combining network I2, such as a hybrid coil, well known in the art. The output of high-pass filter 8 is impressed upon a variable gain-amplifier I4 with its associate control unit I and is then impressed upon the combiningv network I2. The

quency bands. In such a three-band system one frequency band, through a low-pass filter, may extend from zero to 1,000 cycles, the second frequency band through a band-pass filter may exthe compressor unit shown in the above-mentioned Singer patent.

The output of the combining network is impressed upon an amplifier I1 and then on a sound recorder I8 of any standard type. The low-pass filter may be adjusted to pass a band of frequencies in the neighborhood of zero to between 1,500 to 2,000 cycles, while the high-pass filter will pass the remaining frequencies in the audio v spectrum. Thus, the division in the audio frequency spectrum may be somewhere in the neighborhood of between 1,500 and 2,000 cycles when only two mutually exclusive frequency bands are employed although the frequency spectrum may be divided at other points if desired. The variable gain amplifier I0 and control unit II compress the l'ow frequency band to any predetermined extent while the variable sain amplifier I4 and control unit I5 varies the high frequency band ln a predetermined amount.

Ifo illustrate one preferred adjustment of the lunit just mentioned, reference is made to Fig.

3, wherein the relationship between the input and output levels for two frequency bands is illustrated. The straight-line curve shows a linear relationship between input level and out-` put level and which will be followed forv all frequencies of low amplitudes.- The solid curve 2l shows the compression applied to the low fre-y quencies while solid curve 22 shows the compressionapplied to the high frequencies. To obtain curve 2|, the variable gain amplifier I0 and control unit I I are set so that the break-away point occurs at an input level shown at 24, with a compression action which approaches that of a limiter. The variable gain amplifier I4 and lcontrol unit I 5 are adjusted so that the break-away point for the high frequency band occurs at a lower input amplitude level shown-at 25 while the compression rate is more gradual as shown by the curve 2 2. The manner of selection of the breakaway points 24 and 25 and the manner of obtain--l ing the desired degree of compression thereafter tend from 1,000 to 2,000 and a third frequency band, through a high-pass filter, extend from 2,000 cycles upward. With such a band-pass compressor system, each band may have a different rate of compression and starts at a dii.'- ferent amplitude level.

A modification of the system of Fig. 1 is shown in Fig. 2, wherein the output ofamplifier 6 is fed to a limiter 3l, the output of which is impressed upon a low-pass filter 35 and a high-pass is explained .in detail in the above-mentioned Singer patent. y v

Analyzing what occurs with the adjustments shown in Fig. 3. it is to be noted'that compression is applied to the high frequencies at a much lower Input level thanv that at which compression is applied to the low frequencies, so that the Aamplitude ratio of the high frequencies to the low frequencies at any particular instant after compression begins will be less thanv the ratio below the compression level shown at 25. In this manner the intelligibility of reproduction in the.

filter 3B similar to filters 'l and 8 in Fig. l. The output of low-pass filter 35 is impressedupon a standard, amplifier 38 and then upon a combining network 89, while the output of the highpass filter 36 is impressed upon a variable gain amplifier with its associate control unit 6l. The combiningnetwork 39 may be connected to an amplifier similar to amplifier Il of Fig. 1.

In this arrangement the limiter 30 will be adjusted to function at a certain input level and will simultaneously limit both the high and low frequency components of the signal, the low frequency band being given a certain amplification at 38 in accordance with the amplification of the high frequencies before impression on the connecting network 88. 'he high frequency band is given a predetermined compression in addition to the limiting action, to provide an amplitude differential between the high and low frequencies at the higher amplitude levels.

The action of the system of Fig; 2 may also be illustrated in Fig. 3 which shows that the llmiter may be adjusted to start its limiting action at 24 for all frequencies, and the compressor set to start compression of the higher frequencies at the lower input level shown at 25. Thus,the high frequencies will follow the solid curve from 25 to 48 at which level the limiter also affects the high frequency band which is given additional compression, the output level thereof following the dotted curve l2 instead of the solid curve 22, as in Fig.- 1.

From the above description it will be noted that either system of Fig. 1 or Fig. 2 will introduce different amounts of compression between the mutually exclusive frequency bands, the original signal not only' being compressed to within any desired overall range, but the amplitude proportions of the frequencies within the signal are varied 4with amplitude to accomplish the results set forth above.

I claim as my invention:

1. A sound recording system comprising a source ofslgnal currets in the audio frequency spectrum, a plurality of filters for dividing said spectrum into mutually exclusive frequency bands, a separate transmission circuit for each `of said bands, an individual compressor in each of e y,said circuits for each of said frequency bands, a

combining network connected tol said circuits for combining the output of said compressors into a single signal, and a sound recorder connected to aters isimproved as described in mycopending said combining network for recording said sig- 2,395,159 nal currents into a'single record, said combined I signal being. adapted to be reproduced in compressed form.

2. A sound recording system in accordance with gin compressing said frequency bands at diifer ent amplitudes thereof, the rate of compression for each respective band being different to increase the amplitude ratio of the high frequencies to-low frequencies at the higher amplitudes ofw10 said signal currents.

3. The method of varying the amplitude ratio of high frequencies to low frequencies'in an audio signal during transmission and reproduction,

comprising separating the frequencies of said audio signal into mutually exclusive lfrequency bands, introducing compression for each loandv individually at different amplitude levels of the 4frequencies of each band, varying the rate of compression for each individual band in accordance with the amplitude of the respective frequencies in said signal. and combining said compressed bands for reproduction of said audio sig.-

girming at a predetermined amplitude thereof and at a predetermined rate above said predetermined amplitude.

prising a source of said signals, a limiter for limiting the amplitude of said signals above 'a certain predetermined amplitude, a plurality of 1 claim 1 in which said individual compressors be- 6 ters for dividing said signals into mutually exclusive frequency bands, and means for compressing the frequencies in at least'one of said frequency bands, said compression`beginning at a lower amplitude than said predetermined limiter 15 the ratio between the amplitudes of the high and low frequencies of said signals. l

'7.v Ilf'he method of recording an audio signal comprising limiting the amplitude of said signal to a certain. predetermined amplitude separating 2o the frequencies of said limited audio signal into Y mutually exclusive frequency bands, introducingl e compression in each band individually at different amplitude levels of the frequencies of each band, -varying the rate of lcompression for each 25 individual band in accordance with the amplitude of the respective frequencies in said'signal, andcombining said compressed bands for recording intoa single record for direct reproduction without expansion.l

30 8. 'I'he method of recording `in accordance with claim 7 in which the high |frequencyband isk compressed at a higher rate than the low frequency band, at `the higher amplitudes.

5 FREDEiarcKaALBm.

5. A system of compressing audio` signals com- 

